Ultrasonically clearing precipitation

ABSTRACT

A system for clearing precipitation from a window comprises one or more transducers (1-8) fixed to the window. The transducers are driven by a generator (13) to produce surface acoustic waves that propagate through the window. The window may be a laminated window such as a windscreen (10) for a vehicle. A sensing system (122) may be used for detecting the presence of precipitation to actuate the clearing system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/003,516 filed on Jan. 21, 2016, which is a National Phase of PCTPatent Application No. PCT/EP2014/065559 having International filingdate of Jul. 18, 2014, which claims the benefit of priority of UnitedKingdom Patent Application No. 1313061.2 filed on Jul. 22, 2013. Thecontents of the above applications are all incorporated herein byreference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to ultrasonically clearing precipitationfrom a window. Embodiments of the invention relate to clearingprecipitation from a laminated windscreen of a vehicle.

Description of the Related Technology

Conventionally, a driver of a vehicle uses wipers to removeprecipitation from the one or more windows to maintain a clear viewthrough the window. However, the wipers are rubber or plastic andassembled to a metal fixing with a motor and the lifetime of the wipersdepend on how long it takes for the parts to perish. Commerciallyavailable products such as RainX® can be applied to the surface of awindow for easy cleaning of the window. However, since the wiperscontact the surface of the window they also remove products applied tothe window surface when they are in use and further application of theproduct is then necessary.

SUMMARY

According to a first aspect of the invention, there is provided a systemfor clearing precipitation from a window, the system comprising awindow, one or more transducers, and a generator for generating anultrasonic drive signal for the one or more transducers, wherein the oneor more transducers are fixed to the surface of the window and driven bythe generator to produce surface acoustic waves, wherein the surfaceacoustic waves propagate substantially only through a surface region ofthe window.

There is provided a system according to the first aspect of theinvention, comprising a control system having a sensor arranged to senseultrasonic waves emitted by one or more of the said transducers fordetecting the presence of precipitation, and a controller responsive tothe sensor for controlling the operation of the system or apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

Various features and advantages of the present disclosure will beapparent from the detailed description which follows, taken inconjunction with the accompanying drawings, which together illustrate,by way of example only, features of the present disclosure, and wherein:

FIG. 1A is a schematic illustration showing a vehicle having transducerslocated in a peripheral region of a windscreen.

FIG. 1B is a schematic illustration showing a windscreen and electronicsfor operating the transducers.

FIG. 2 is a schematic illustration showing a cross-section through avehicle windscreen with a transducer bonded to its surface.

FIG. 3A is a schematic illustration showing a transducer emittingsurface acoustic waves into precipitation on the surface of awindscreen.

FIG. 3B is a schematic illustration showing an angled windscreen with atransducer emitting surface acoustic waves into precipitation on thesurface of the windscreen.

FIG. 3C is a schematic illustration showing droplet propulsion andatomisation of precipitation using surface acoustic waves.

FIG. 4 is a schematic illustration showing the contact angle ofprecipitation for hydrophobic and hydrophilic coatings.

FIG. 5 is a schematic illustration showing surface acoustic wavesemitted from a transducer in a pulsed mode.

FIG. 6 is a schematic illustration showing a method for matchingimpedance lines in the system.

FIG. 7A is a schematic diagram showing a design for an inter-digitaltransducer for operation at a frequency of 500 kHz.

FIG. 7B is a schematic illustration showing the wavelengths of differenttypes of waves at a frequency of 500 kHz through glass 3 mm thick.

FIG. 7C is a schematic illustration of different wave types.

FIG. 8 is a graph showing the calculated wave speeds as a function offrequency for different types of waves traveling through 3 mm thickautomotive glass.

FIG. 9A is a schematic illustration showing a design for a transducerfor operation at a frequency of 1 MHz.

FIG. 9B is a schematic illustration showing a design for a transducerfor operation at a frequency of 500 kHz.

FIG. 10 is a schematic illustration of a visor having transducersattached to a visor.

DETAILED DESCRIPTION OF CERTAIN INVENTIVE EMBODIMENTS

In the description, the term “acoustic wave” is used to refer to a waveproduced by a transducer that is being driven; it does not refer to thefrequency of a wave being in the audible acoustic range for people.

Precipitation includes rain, sleet, snow, ice, drizzle, mist, fog, hailor other types of precipitation. When precipitation falls onto a windowof a vehicle, for example the windscreen, it impedes the view for adriver.

When precipitation falls onto a window it is attracted to the surface ofthe window by surface tension. The precipitation, for example liquidwater, can form many droplets across the window surface. The applicant'sresearch has found that each of the many droplets will be a differentsize, have a different diameter and have a different shape which may beregular or irregular. An example of a droplet size may be approximately0.4 millilitres (ml) having a diameter of approximately 1 centimetre(cm) for example but could be much smaller. The front window orwindscreen of a vehicle such as a car is inclined, for example, at anangle of 34□. The angle may be greater. Some vehicles have windscreensinclined at a greater angle, for example 35□ or more. A rear window maybe inclined at a larger angle than the front windscreen. Large liquiddroplets will run down the windscreen faster than small liquid dropletsdue to their larger mass and greater influence under gravity. Othereffects such as surface tension of the droplet and air flow over thesurface can affect how a droplet moves across the surface. Surfacetension may affect small droplets more than larger droplets. As thedroplet size decreases, the internal pressure of the droplet increases.For example, smaller droplets require a larger angle before running offthe windscreen compared to larger droplets. In an illustrativeembodiment, the merging of droplets using ultrasonic waves duringoperation of the system may be useful since the larger droplets may bemore easily cleared from the windscreen compared to smaller droplets dueto their larger mass and influence under gravity and airflow. Inaddition, the surface tension effects without air flow are constant andmay be independent of temperature.

Embodiments of the present invention use ultrasonic waves to removeprecipitation from the surface of a window. In general, ultrasonic wavesare acoustic waves with a frequency above 100 kiloHertz (kHz) and up toaround 50 MegaHertz (MHz) or higher. The ultrasonic waves used inembodiments of the invention have a frequency in the range of about 400kHz to 1.5 MHz. Transducers are used to produce acoustic waves in arange of frequencies. The transducers do not operate at a singlefrequency but instead operate across a range of frequencies (i.e.bandwidth) either side of a central frequency. The operating frequencyof a transducer is to be understood as relating to the main operatingfrequency or central frequency of the transducer within the bandwidth offrequencies.

In embodiments of the invention, transducers are bonded to a windowsurface and driven to emit acoustic waves having ultrasonic frequencies.The range of frequencies of acoustic waves emitted from the transducerare dependent on the design of the transducer.

FIG. 1A shows an embodiment of the invention wherein transducers (1-8)are positioned along the peripheral area (9) of a windscreen (10) of avehicle. The transducers are bonded or glued to the periphery of thewindscreen. There are limitations on the attachment locations of thetransducers for droplet removal from a windscreen. The transducers mustbe positioned so as not to obstruct the view of the driver or otheroccupants of the vehicle. The position of the transducer on thewindscreen may influence the efficiency of the transducer in terms ofability to clear precipitation from the windscreen. Any suitable numberof transducers may be used for removing precipitation from thewindscreen. There may be a plurality of individually spaced transducersalong the peripheral area of one or more sides of the window orwindscreen. The transducer may also be arranged to form a continuousstrip either at the sides or top and bottom of the windscreen, or boththe sides and the top and bottom. An inter-digital transducer (IDT) maybe used.

FIG. 1B schematically shows the driving electronics for the transducers.The transducers are connected to a driving electronics system via wiring(123) where the driving system comprises a power supply (11), a controlunit (12), a frequency generator (13), a power amplifier (14), and apulse generator (15). The power supply may be a 12V or 24V vehiclebattery. The driving system may be controlled by a rain sensor (122)and/or other manual controls (121). The rain sensor can be a proprietyitem or can be formed using the transducers already part of the systemwith the appropriate additional circuitry.

The transducers are bonded to the windscreen and energised or driven bythe driving electronics. Suitable bonding agents are commerciallyavailable and are used to fix each transducer to the windscreen. Thebonding agent is used to form a uniform bonding layer between eachtransducer and the surface of the window. In an illustrative embodimentof bonding the transducers to the windscreen, the bonding agent is mixedin a vacuum to prevent air bubbles forming within the bonding layer. Ifgas bubbles are present in the bonding layer, ultrasonic frequencieswill be highly attenuated and it could impede the efficiency of thetransducers. An example of a suitable bonding agent is epoxy resin. Inan embodiment, the epoxy resin may be prepared or provided in a vacuumbag ready for mixing prior to application to the windscreen andtransducers, wherein the vacuum bag comprises two compartments separatedby a barrier and wherein the barrier is broken in order to mix the epoxywithin the vacuum bag. In an embodiment, the bonding layer is thin tominimise the refraction of sound through the multi-layered system of theglass, bonding layer and transducers. The bonding agent may have otherspecial properties such as acoustically matching the impedance of thebonding agent to the impedance of the window surface to which they arebeing attached, in order to efficiently couple or transmit acousticwaves into the window by minimising unwanted reflections from the windowsurface. Each transducer comprises a set of electrodes as the activeelements next to a piezoelectric layer, and a ground electrode. In someembodiments the transducers are attached with one electrode (forexample, a ground electrode) facing away from the window and oneelectrode (for example, a cut electrode) adhered to the outer surface ofthe windscreen. In an embodiment, the transducers are bonded to thesurface such that the transducer surface is parallel with the surface ofthe windscreen or other surface to which they are being attached.

Each transducer is driven by the frequency generator 13 and poweramplifier 14 of FIG. 1B to emit a range of frequencies. The range offrequencies or bandwidth of frequencies emitted may be fixed and chosenby the designer. Alternatively the frequencies or bandwidth may bechosen by an operator. The operator may be, for example, a driver of thevehicle in which the transducer is installed. For example, the drivermay have the option to select the range of frequencies emitted accordingto the amount of precipitation to be removed from a window, such as forheavy rain or light drizzle conditions. This can also be conductedautomatically using a rain sensor. This may take the form of a dial orbuttons for the driver to select within the vehicle according to theconditions. Driving the transducer causes the transducer to emitacoustic waves. The transducer design may determine the full range ofoperating frequencies that the transducer is capable of being driven toproduce. The acoustic waves emitted depend on factors such as transducerdesign, contact angle between the transducer and surface to which thetransducer is bonded, driving power, among other factors. The frequencyand dimensions of a transducer may be chosen to affect the spread of theemitted acoustic beam from the transducer, for example, the higher thefrequency selected the more focussed the emitted acoustic beam may be.The wavelength of each type of acoustic wave emitted is a function ofthe spacing between the electrodes of the transducer.

The transducers can be driven in continuous or pulsed mode. A pulsedgenerator can be used to drive the transducers in a pulsed mode. Inpulsed mode the acoustic waves will be emitted from the transducer inpulses. The frequency generator may provide frequency modulated signalsto produce frequency modulated acoustic waves. In an example, thefrequency of the waves is driven through a range of frequencies byfrequency sweeping.

Each wave consists of nodes and antinodes—nodes are regions of a wavehaving minimum amplitude and antinodes are regions of a wave havingmaximum amplitude. Standing waves occur when there is a stablesuperposition of waves in a system. For example, a transmitted wave andreflected wave may combine to form a standing wave due to cancellationor amplification of their frequency components. In an example, a wavetraveling along the surface of the window may be reflected at the windowedge due to an acoustic impedance mismatch between the window materialand the surrounding medium. The reflected wave can interfere with thewave traveling in the opposite direction such that the phases of the twowaves cancel each other out or combine to cause a standing wave to form.The applicant's research has found that, in an example, droplets sittingon a windscreen surface will feel the influence of acoustic wavestraveling through or along the windscreen. The droplets may be observedto vibrate or move along the windscreen at different speeds which maydepend upon the positions of nodes or antinodes of the waves travelingthrough or along the windscreen. When the transducers bonded to theperiphery of the windscreen are driven, there may be a distribution ofultrasonic vibration in the windscreen, for example, the presence ofmaxima and minima corresponding to a spatial interference pattern.Droplets moving at a greater speed compared to other droplets may becaused by regions of the windscreens at which antinodes are located orareas close to where the transducers are located. Vibrating droplets orslow moving droplets may be located at or near a node on the windscreenor further from where the transducers are located. Droplets locatedclose to the transducers may experience a direct sound field whereinsurface acoustic waves (SAWs) are emitted and encounter a droplet beforethey have been reflected somewhere in the windscreen. Droplets locatedat greater distances from where the transducers are located may bemainly subjected to a reverberant energy field wherein ultrasonic wavesmay encounter the droplet from all directions or may be reflected atboundaries before encountering a droplet. Using pulsed energy may reducethe level of the reverberant field, such as for the embodiment of FIG.5.

In other examples, droplets may vibrate and cause smaller droplets tocombine with other/separate droplets to form larger droplets, which maythen run off the surface of the windscreen removing the precipitation.

FIG. 2 schematically shows a cross-section through an example of thewindscreen (10) of the vehicle of FIG. 1A. The windscreen consists of alaminate layer (20) sandwiched between two sheets of glass (21-22). Theglass is suitable for automotive use. Due to current safety legislationmeasures, it is required that a vehicle windscreen be laminated, where apolyvinyl butyral (PVB) laminate is compressed between two layers ofannealed glass. A nominal amount of adhesive may be sprayed onto theglass surface and heat applied to compress the laminate layer betweenthe layers of glass. This reduces the problem associated with thedestruction of a single layer of tempered windscreen glass on impact,such as during an accident, which were previously used. The laminatedglass also serves to contain the passenger airbag within the cabin areaof the vehicle in such circumstances. The laminate layer is areinforcing layer that may be 0.38 millimetres (mm) thick and the sheetsof glass either side of the laminate layer may each be 3 mm thick suchthat the total thickness of the windscreen may be around 6.4 mm thick.There may or may not be an optional coating layer (23) on the topsurface of the windscreen. A transducer (1-8) is bonded to thewindscreen near the edge of the windscreen or in a peripheral region ofthe windscreen. A bonding layer (24) attaches/fixes the transducer tothe surface of the windscreen. The transducers may be affixed to thewindscreen and concealed from external view. The transducers may behidden below a rubber or plastic seal (25) which surrounds mostconventional windscreens and runs along the periphery of the windscreen.The transducers are positioned so that the operation of the transducersis not affected by the presence of the rubber seal. The windscreen hasan outer region (27) of the windscreen which is exposed to the weather,and an inner region (26) which relates to the interior of the vehicle towhich the windscreen is fitted.

FIG. 3A is a schematic showing a transducer of FIG. 1A, 1B or 2 bondedto the surface of a windscreen which has precipitation on its surface.In this example a water droplet (30) is present on the surface of thewindscreen. The transducer is driven to produce waves (31) that travel(39) along the surface of the windscreen as shown, also referred to asSAWs. The frequencies of the SAWs emitted are in the range 400 kHz to1.5 MHz. and preferably the main operating frequency is 1 MHz. Eachtransducer may have a main operating frequency within a bandwidth offrequencies. For example, a transducer designed to have a main operatingfrequency of 1 MHz may also be driven to operate at 500 kHz within itsbandwidth of frequencies. However, a transducer having a main operatingfrequency of 1 MHz that is driven at 500 kHz may not perform asefficiently as a transducer having a main operating frequency of 500 kHzand driven at 500 kHz. The SAWs are emitted from the transducer. Thelaminate layer within a vehicle windscreen is highly attenuating toultrasonic waves and causes a damping effect. Therefore the ultrasonicwaves may be confined to a region at the surface such that they do notpenetrate deeply into the glass or the laminate layer. The wavefrequency and mode of the transmitted wave may be chosen so as not tointeract substantially with the plastic or laminate layer. Therefore,the SAWs are coupled to the surface of an object, or windscreen, and maynot penetrate into the laminate layer within the windscreen. Suitablewaves for this application may include Lamb waves. Rayleigh waves orother shear-type waves. Other types of waves are likely to be attenuatedand dissipate their energy into the laminate layer. In addition to SAWstraveling at the surface of the windscreen, other waves (32) may also beemitted into the body of the windscreen. The other waves emitted mayinclude longitudinal or shear-type waves. In some embodiments, thelaunch angle of the waves into the top layer of automotive glass may bechosen to produce the desired type of ultrasonic wave traveling throughthe glass.

A calibration of the transducers may be performed to optimise theoperating efficiency of the system.

The SAWs will be coupled to the surface of the windscreen and onreaching the edge of the windscreen will be partially reflected (33)back into the windscreen along its surface as shown in FIG. 3A. Thereflections will be due to an acoustic impedance mismatch between theglass material of the windscreen and the medium of the outer region orperipheral region (34) of the windscreen. For the frequencies ofinterest in the range 400 kHz to 1.5 MHz, the ultrasonic waves will behighly attenuated through air.

The applicant's research has found that when the SAWs encounter a waterdroplet, some of the wave energy will be transferred to the droplet viamode conversion (35) and longitudinal waves (37) may travel through thedroplet. When ultrasonic waves or SAWs are applied to water droplets,the droplets may be atomised (also known as jetting or vaporisation).There are three progressive stages that can be observed includingstreaming (36), propulsion and atomisation.

The contact angle (38) of the water droplet to the surface will affectthe angle at which the SAWs will encounter the droplet. This angle hasbeen labelled as □R and may relate to the Rayleigh angle of Rayleighwaves traveling along the surface of the windscreen. The SAWs may beRayleigh waves or Lamb waves. If the SAWs are Lamb waves, these mayrelate to anti-symmetric Lamb waves or a flexural mode. As the SAWsenter the droplet, mode conversion takes place and longitudinal wavesare transmitted into the water droplet. Due to the mode conversion andtransfer of energy the SAW amplitude decreases and may also be referredto as a “leaky” wave (43). The longitudinal waves transmitted into thedroplet causes streaming to occur within the droplet whereby internalrotational mixing and some cavitation takes place. The next stage isexhibited by “propulsion” of the droplets, where they move rapidly atright angles to the transducers or IDT electrodes.

FIG. 3B is a schematic showing the front windscreen on the vehicle ofFIG. 1A or 1B which is inclined at an angle, in this embodiment theangle is □=34□. In some embodiments of the invention the surface of thewindscreen may be curved. The applicant's research has found that whendroplets are propelled (40) to move along a surface, each water droplethas a leading edge (41) at the front and a trailing edge (42) at theback. The leading edge has a different shape to the trailing edge. Thecontact angle of the droplet with the surface is different for theleading edge and trailing edge. As discussed above, the contact anglesat the edges of the droplet depend upon the surface treatment of thesurface on which the droplets are sitting or moving across. For waterdroplets moving on an inclined surface the water droplet will have adifferent contact angle (38) with the surface at the advancing edge andtrailing edge when compared to the contact angles of a water droplet ona flat surface. As shown a SAW emitted from the transducer is coupledinto the droplet at the angle □R. In this example the SAW encounters thedroplet at the trailing edge, however other SAWs may encounter thedroplet at the leading edge due to being emitted from a differentlocation around the periphery of the windscreen, or due to reflections.The other SAWs encountering the droplet at the leading edge will becoupled into the droplet at a different angle than □R.

In the example of FIG. 3C, a droplet is shown as being vaporised oratomised (44). SAWs traveling along the windscreen are able to transferenergy into the droplet causing the droplets to vibrate or resonate (45)or move along the surface. With enough energy the droplets may resonate,causing internal rotational mixing and cavitation, and burst into manysmaller droplets (46) thereby being atomised. As such, the precipitationis cleared or removed from the surface of the windscreen.

The internal pressure changes within the droplet cause the droplet shapeto change. The droplet shape changes by becoming skewed (48) along thedirection in which the SAWs travel as shown in FIG. 3C. In this Figure,the droplet is shown on a level surface to highlight the effect that theSAWs have on the shape of the droplet. However, in other embodiments thedroplet shape may also be effected by the angle of inclination of thesurface. When SAWs are applied to the droplet, firstly, the droplets areseen to “stream” wherein the internals of each droplet rotates, possiblycaused by cavitation. Secondly, the droplets undergo propulsion and maybe observed to vibrate or be propelled to move across the surface of thewindscreen. Applying SAW energy may allow the droplet to overcomesurface tension and move or become propelled along the surface of thewindscreen. The direction of propulsion of the droplet may be in thesame direction in which the SAWs travel. The driving power of thetransducers may be increased so as to effectively propel the dropletsacross the surface of the windscreen. During the “propulsion” processthe droplets may collide with one another and form larger droplets,which due to their greater mass and reduced surface tension, may movemore rapidly across the windscreen. The mechanism of propulsion may bedue to streaming associated with the creation and collapse of cavitiesinside the liquid or due to “bubble activity” (47) arising fromcirculation of the liquid inside the droplet. Thirdly, the droplets maybe atomised wherein the droplet is split into much smaller droplets. Inthis way precipitation may be cleared from a windscreen using SAWs.

The applicant's research has found that since each droplet will have adifferent size with varying diameters, each droplet will have adifferent resonant frequency and may vibrate at a preferred frequencywithin a range of frequencies. When a SAW with the correct frequency tomatch that of the resonant frequency of the droplet is encountered, thedroplet will resonate in a high energy state. To “hit” the resonantfrequency of each droplet the transducers may be driven to sweep througha range of frequencies. The many SAWs traveling through the windscreen,including reflected waves, may combine via superposition to locallyincrease their amplitude at some frequencies. The driving frequency ofthe transducers can be varied by sweeping through a range offrequencies. In an embodiment, this enables the SAW frequency to “hit”the resonant frequency of a droplet. In this way, the differently sizeddroplets may be vaporised. As the droplet size decreases, its internalpressure increases. It may therefore be easier to vaporise smallerdroplets than larger droplets.

As mentioned earlier, a droplet size may be around 0.4 ml with adiameter of around 1 cm. For droplet resonance to occur an integernumber of half-wavelengths may need to fit within the droplet diameter.For example, for a SAW having a wavelength of 1 cm and a dropletdiameter of 1 cm, there will be two half-wavelengths of the SAW that mayfit within the droplet to cause resonance of the droplet.

In other examples the precipitation may not be a water droplet but mayinstead be hail, snow or a layer of ice or other precipitation. Forprecipitation other than water droplets such as for rain, theapplication of the ultrasonic waves may vary to achieve a similar effectof clearing the precipitation from a window. For example, ultrasonicwaves may be employed to break down a sheet of ice or frost formed overthe windscreen during winter.

As discussed, to further improve the process of clearing precipitation,the outer surface of the windscreen can be treated with an optionalcoating. FIG. 4 is a schematic showing the effect that an optionalcoating layer (23) on the surface of the windscreen may have on adroplet on its surface. This optional coating will be located on thesurface of the windscreen at the outer region. For example, the optionalcoating (23) on the outer surface may be a hydrophobic coating (49)which may be added by spraying or wiping onto the surface. A hydrophobiccoating causes any water droplets on the layer to be repelled from itssurface so as to minimise the contact area of the water droplet with thehydrophobic coating. In an illustrative embodiment a hydrophobic coatinglayer on the windscreen is preferred to aid the removal or clearing ofprecipitation from the windscreen surface. In an example of a transduceremitting SAWs across a hydrophobic coating, the contact angle □R1 ofSAWs with the droplet will be large. Alternatively, the optional coating(23) may be a hydrophilic coating (50). If a hydrophilic coating (50)were applied to the surface of the windscreen, the contact area of thewater droplet will be much larger and the contact angle □R2 of SAWs withthe droplet will be small. For removing precipitation from the surfaceof the windscreen, a hydrophobic coating layer is preferred over ahydrophilic coating layer. Since the contact angle of the SAWs with thewater droplet is larger for a hydrophobic layer than a hydrophilic layer(□R1>□R2), there may be more efficient mode conversion of the SAWs intothe droplet and the droplet may be propelled across the surface of thewindscreen more effectively. A hydrophobic coating reduces surfacetension by changing the contact angle between the droplet and thewindscreen surface. The air flow over the surface of the windscreen mayalso assist in removing precipitation from the windscreen. The contactangle between a droplet and a surface will also depend upon theviscosity of the droplet and the type of material of the surface. Forexample, whether the droplet is water or oil, or the surface isautomotive glass or a plastic or polycarbonate material such as are usedin visors for motorcycle helmets.

FIG. 5 is a schematic showing the transducers of FIGS. 1A to 3Boperating in pulsed mode. The transducers may be driven in pulsed modesince pulsing the waves (51), for example at half-second intervals, maystop the temperature of the transducers rising too much because itreduces the build-up of heat in the system. Thus allowing for a higheramplitude of input signal to remove precipitation more rapidly.

Frequency modulation may be employed to more effectively clearprecipitation than amplitude modulation which may not be as effective.The transmission efficiency may also be optimised by acoustic impedancematching.

The power efficiency of the system may be optimised, since about twothirds of the energy can be transferred or lost as heat. Acoustic lossesmay include scattering or absorption within the system, for example atglass impurities or defects. To prevent heating effects, the circuitryand materials such as the transducers on the window may be optimised forimpedance matching.

FIG. 6 schematically shows an impedance matching circuit (60) whichmatches the electrical impedance of the power amplifier (14) of thedriving circuit to the impedance of the transducers (1-8). This improvesthe efficiency of the power circuit or system as a whole and reducesloss of energy from the system. There may be unwanted reflectionsarising from a mismatched impedance line.

Other impedances may be matched or improved matching obtained byminimising the acoustic impedance difference between the transducer andthe surface to which the transducer is bonded. An anti-reflectioncoating can be used on the surface of the transducer to enhance thecoupling of ultrasonic waves from the transducer into the surface towhich it is bonded. The transducer design may be optimised to minimisethe acoustic impedance mismatches between surfaces such as to maximisethe coupling of waves.

Two types of transducer design will now be discussed for square andcircular transducer designs. Many piezoelectric transducers arecommercially available in the circular form. However, the circulardesign is not favoured in this application because the circulartransducer design radiates acoustic energy equally in all radialdirections. The square form is preferred because it radiates acousticenergy in directions that are perpendicular to its electrodes. Thereforethe acoustic energy may be more closely controlled during application toa windscreen for removing precipitation. The cutting or shaping of atransducer may change its resonant frequency. In an illustrativeembodiment, the electrode finger spacing of the IDT may be adjustedaccording to the characteristics of the transducer and the windscreen.

FIG. 7A is a schematic diagram showing an embodiment for one possibledesign for a transducer (1-8) of FIG. 1A or 1B, appropriate foroperation at 500 kHz. The transducer shown is an inter-digitaltransducer (IDT). An IDT may be fabricated from piezoelectric materialor by modifying the outer electrode of a standard piezoeletrictransducer by cutting through the electrode and leaving thepiezoelectric material uncut as much as possible. IDTs are designed towork by matching the spacing (71) of the electrode fingers to thewavelength of the waves that may be needed to be excited, depending onthe application at hand. This may correspond to the frequency at whichthe transducer resonates. In this way it is possible to fabricate an IDTwhich can generate SAWs at ultrasonic frequencies. The IDT caneffectively be tuned to match the physical constants of the outer glasslayer of the windscreen to optimise the efficiency of the system. Thetransducer design shown may produce waves that penetrate to a depth ofless than 3 mm from the surface of the windscreen or windshield. Thiswill prevent the waves from suffering damping effects due to thelaminate layer which is located at a depth of 3 mm into the windshield.

For the transducer in FIG. 7A, the dimensions of the transducer areshown. The dimensions of the electrodes may be specifically chosen toselect an operating frequency for the transducer. In this example, thediameter (72) of the transducer is 40 mm and the first (73) and secondelectrodes (74) are separated by a gap (75) between the electrodes of 4mm. The second electrode (74) is shown to be 11 mm wide, giving anelectrode finger spacing (71) of 15 mm. The electrode shown operates at500 kHz.

FIG. 7B is a schematic illustration showing the wavelengths of differenttypes of waves emitted from the transducer design of the embodimentshown in FIG. 7A for 3 mm thick automotive glass. For an electrodefinger spacing of 15 mm, the wavelength of Lamb waves (76) (or Rayleighwaves) emitted will be around 7.8 mm, longitudinal waves (77) will be10.8 mm and shear waves (78) will be around 7.0 mm. The wavelengths ofthese three types of wave emitted will be larger than the 3 mm thicknessof the glass through which they will be traveling. The phase velocity orspeed of these waves in automotive glass can be calculated using thewell-known acoustic equation (c=f□, for phase velocity, c=frequency,f×wavelength, □). In this example, the Lamb waves travel at around 3900metres per second (m/s), the quasi-longitudinal waves at around 5400 n/sand the shear waves at 3500 m/s. The spacing of the electrode fingers isroughly twice the wavelength of the Lamb waves. The electrode fingerspacing is around 3/2 wavelengths for the longitudinal waves such thatthe longitudinal waves are not excited in this particular embodiment ofthe invention.

FIG. 7C shows examples of the types of waves that may be emitted fromthe different transducer designs. Other waves may also be emitted, forexample Rayleigh waves, longitudinal waves or shear waves.

The speed (and hence wavelength) of waves in the automotive glass of thewindscreen may vary with frequency, material properties (for example,Young's modulus, density, or Poisson ratio), and thickness of the glass.These parameters may be known within a certain tolerance orexperimentally measured. For example, a laser vibrometer may be used toaccurately determine the spatial field of vibration within thewindscreen during the operation of the system for the purposes ofobtaining a more accurate measure of the wave speeds within the glassfor refining and improving the efficiency of the transducer designs.

FIG. 8 shows a graph of calculated phase velocities (80) of each type ofwave (76-78) as a function of the wave frequency (81) through 3 mm thickautomotive glass. These calculations are based on the assumption thatthe material through which the waves are travelling is a thin platehaving the following parameters: thickness of glass=3 mm; Young'smodulus of glass=70 GigaPascals (GPa); density of glass=2500 kilogramsper metre cubed (kg/m3); and Poisson ratio of glass=0.23. The Lamb waveshown may be slightly high and relate to an anti-symmetric Lamb wave orflexural mode. The phase velocity of the longitudinal and shear waves ina chosen direction through automotive glass are relatively constant (82)at all frequencies shown. The phase velocity of the Lamb (or Rayleigh)waves increases (83) with higher frequencies. As may be seen, the shearwave speed is almost identical to the wave speed of the Lamb waves (orRayleigh waves) at 500 kHz. This may be beneficial for effectivelyclearing precipitation from the surface of the windscreen.

Some square transducer designs have also been used in certainembodiments of this invention. The square transducers used are 2 cm by 2cm and are fabricated from standard piezoelectric, such as leadzirconate titanate (PZT) material. To form the electrodes of thetransducer, grooves are mechanically or laser cut into the piezoelectricmaterial. All of the electrodes will be operated simultaneously usingthe same electrical signal.

FIG. 9A and FIG. 9B show two square transducer designs usable for thetransducers of FIG. 1A or 1B. Transducers designed to operate at higherfrequencies, for example at a frequency of around 1 MHz, may be smallerand generate shorter wavelength SAWs compared to transducers designed tooperate at lower frequencies, for example at a frequency of around 500kHz, which may be larger and generate longer wavelength SAWs. Thepreferred operating frequency may be 1 MHz and may be for a transducerhaving a square design (as opposed to a transducer having a circulardesign). In some embodiments, the thickness of the transducers are a fewmillimetres thick and the larger the area of the transducer the thickerthe piezoelectric layer. The electrode layer sits on top of apiezoelectric material and the electrode layer is much thinner than thepiezoelectric layer. In some embodiments the electrode layer is a thinfilm much less than 1 mm thick. The electrode layer may be cut using alaser or cut mechanically, however cutting using a laser may provide abetter finish to the electrode fingers than mechanical cutting which mayleave burred edges on the electrodes.

FIG. 9A shows a transducer design for a piezoelectric material (90)thickness of d=3 mm, having electrodes (91) that are 1.35 mm wide whichcorresponds to an operating frequency of 1 MHz (for a wavelength of 2.7mm at a phase velocity of 2700 m/s), for an area of 2 cm by 2 cm. Inthis example, the electrodes (91) are adhered to the surface of thewindscreen and the uncut side of the piezoelectric material faces awayfrom the windscreen. Each electrode finger width corresponds to half ofthe acoustic wavelength for each of the emitted waves. Other similarexample transducer designs may have electrodes that are 1.08 mm and 0.9mm wide for operating frequencies of 1 MHz (for a wavelength of 2.16 mmat a phase velocity of 2160 m/s) and 1.2 MHz (for a wavelength of 1.8 mmat a phase velocity of 2160 m/s) respectively.

FIG. 9B shows an alternative transducer design for an IDT. This is a 2.8cm by 2.8 cm transducer design for a square transducer which may befabricated from a circular piezoelectric material (90) (PZT material),as shown on the left of FIG. 9B. The piezoelectric layer in this exampleis 4 mm thick. In this example, the cut electrodes are adhered to thesurface of the windscreen with the other side, or uncut side of thepiezoelectric layer, facing away from the windscreen. A ground electrodemay face away from the windscreen without being adhered to thewindscreen surface. The gaps (93) between the electrodes (92) may be cutusing a high powered laser. The electrodes sit on top of thepiezoelectric material. In this example, the gaps between the electrodesare 0.4 mm wide and resemble a square waveform pattern (95) across theelectrode layer. Cutting through the electrode layer from top to bottomwill form two separate parts of the transducer, each designed for anoperating frequency of 500 kHz (for a wavelength of 4.32 mm at a phasevelocity of 2160 m/s). In this example it may only be necessary to cutthrough the thin electrode layer without cutting into the piezoelectriclayer, provided that a gap is created in the electrode layer to producethe two separate electrodes. The electrodes of the two parts are able toslot into one another. Each electrode has an opposite polarisation tothe other. When the electrodes are combined by slotting into one anotherthe adjacent electrodes produce alternating polarisations (96) from oneelectrode to the next. Electrodes may be oppositely polarised toadjacent or other electrodes so that one of the electrodes is energisedwith one polarity of signal and the other electrode with the oppositepolarity. An example of the droplet motion (94) that may be observedrelative to the transducer electrodes is shown.

The example transducer designs described may be capable of vaporisingdroplets of precipitation on a windscreen or other glass surface.

FIG. 3A has been shown with a laminate layer, however in other examplesa laminate layer may not always be present, such as for a motorcyclehelmet visor. In this example, the visor may be fabricated from aplastic or polycarbonate material. The chosen operating frequencies ofthe transducers for clearing precipitation from the visor in thisexample, may not be the same as the frequencies used in the embodimentsdiscussed for clearing precipitation for the windscreen. For example,the frequencies of the transducers in some embodiments for clearingprecipitation from a visor may be lower than those used for clearingprecipitation from a windscreen.

FIG. 10 shows an embodiment of the invention in which transducers (1-8or 31-33) are attached to a visor (30), for example a visor of amotorcycle helmet or other helmet, for clearing precipitation or otherdebris or material. In the example of a visor, the transducer may befixed directly to the surface of the visor and operate in a similarmanner as described above (for embodiments relating to a laminatedwindscreen in a vehicle), wherein SAWs are used to remove precipitationfrom the surface of the visor. The transducers may be driven by a drivesystem (11-15). In some embodiments the transducers are driven tooperate at frequencies within the range of 100 kHz to 1 MHz. Thetransducers are connected to the drive system by electronic wiring (35)and a plug and socket (36). The transducers may be bonded directly tothe visor or bonded to a removable clip (34) adapted to clip onto theedge of the visor, thus allowing for a change or replacement of adamaged visor. Similarly, the transducers are positioned in a peripheralregion of the visor so as not to obscure the rider's vision. The visorshown does not contain a laminate layer and therefore any suitable wavemay be used for clearing precipitation including SAWs. The waves may becoupled to the surface of the visor or contour of the visor so as to beeffectively transmitted through the entire surface of the visor.

Other examples where a transducer and driving system may be used includea detection system for detecting the presence of rain drops orprecipitation and therefore initiate the system for clearing theprecipitation. In an example detection system two or more transducersmay be employed. A first transmitting transducer may emit ultrasonicwaves to a second receiving transducer. The receiving transducer may beable to monitor the energy of the ultrasonic waves received from thetransmitting transducer. If a calibration is performed when noprecipitation exists on the surface being investigated, there will be abase level of acoustic energy received at the receiving transducer. Whenprecipitation is present on the surface the precipitation will absorbacoustic energy and the receiving transducer will observe a drop in theacoustic energy received below the calibrated base level, thereforeindicating the presence of precipitation. At this point, the ultrasonicsystem for clearing precipitation from the windscreen surface may beswitched on and the ultrasonic power can be automatically changed inline with the severity of precipitation. In this way, the transducersused for operating the system can also be used for controlling it.

Some embodiments provide the advantage of improving the power efficiencyof the transducers or increasing the ultrasonic wave energy in thewindscreen. The system may become more effective and efficient bycarefully “tuning” the IDT. For example the IDT impedance may be matchedto the windscreen or glass, or the input frequency may be pulsed. Inother embodiments, several frequencies may be used to overcome standingwaves on the glass, for example by sweeping through a range offrequencies, or using frequency modulation. In other illustrativeembodiments bending waves or Lamb waves may be found to be moreeffective for inducing “streaming” or propulsion of a droplet, whilstminimising the amount of shear waves emitted which may reduce theeffectiveness of the system.

Other embodiments provide the advantage of minimising the heating of thetransducer, for example by using pulsed waves, which in turn allows formore power to be supplied to the system to allow for a greater area ofthe windscreen to be cleared of precipitation.

The use of IDTs and SAWs has the advantage of minimising any dampingeffect that may be caused by the existence of a laminate layer. This mayminimise issues of providing enough power for droplet removal from awindshield without causing the internal laminate layer within thewindshield to delaminate.

Other advantages of illustrative embodiments may be that the hydrophobiccoating is not be removed or wiped off since ultrasonic transducers areused to clear precipitation from a surface that has been treated andthere are no visibly moving parts across the surface of the windscreen.

Embodiments of the invention may be applied not only to laminatedautomotive windscreens and visors but also to laminated windows ofbuildings and to laminated windows used in any other situation, forexample ships and boats.

Embodiments of the invention may also be applied to un-laminatedwindows.

The preceding description has been presented only to illustrate anddescribe examples of the principles described. This description is notintended to be exhaustive or to limit these principles to any preciseform disclosed. Many modifications and variations are possible in lightof the above teaching.

What is claimed is:
 1. A system for clearing precipitation from awindow, the system comprising: a laminated window; one or moretransducers; and a generator for generating an ultrasonic drive signalto drive the one or more transducers, wherein the one or moretransducers are fixed to a surface of a top layer of glass and driven bythe generator to produce ultrasonic waves such that the ultrasonic wavespropagate substantially only through the top layer of glass.
 2. A systemaccording to claim 1, wherein the window is a laminated windowcomprising a laminate layer sandwiched between a top and bottom layer ofglass.
 3. A system according to claim 1, wherein precipitation includesrain, sleet, snow, ice, drizzle, mist, fog, hail or other types ofprecipitation.
 4. A system according to claim 1, wherein each of the oneor more transducers are configured to operate in the frequency range of400 kHz to 1.5 MHz.
 5. A system according to claim 1, wherein thegenerator comprises a pulse generator and wherein the ultrasonic wavescomprise pulsed ultrasonic waves.
 6. A system according to claim 1,wherein the generator is configured to cause the one or more transducersto sweep through a range of frequencies.
 7. A system according to claim1, wherein the generator is configured to cause the transducer toproduce waves ultrasonic waves which are frequency modulated.
 8. Asystem according to claim 1, wherein the generator is configured todrive the one or more transducers to produce waves ultrasonic waves suchthat mode conversion of the ultrasonic waves propels the precipitation.9. A system according to claim 1, wherein the generator is configured todrive the one or more transducers to produce waves ultrasonic waves suchthat mode conversion of the ultrasonic waves vaporises theprecipitation.